A goal of many modern long haul optical transport systems is to provide for the efficient transmission of large volumes of voice and data traffic over trans-continental distances at low costs. Various methods of achieving these goals include time division multiplexing (TDM) and wavelength division multiplexing (WDM). In time division multiplexed systems, data streams comprised of short pulses of light are interleaved in the time domain to achieve high spectral efficiency, high data rate transport. In wavelength division multiplexed systems, data streams comprised of short pulses of light of different carrier frequencies, or equivalently wavelength, are co-propagated in the same fiber to achieve high spectral efficiency, high data rate transport.
The transmission medium of these systems is typically optical fiber. In addition there is a transmitter and a receiver. The transmitter typically includes a semiconductor diode laser, and supporting electronics. The laser may be directly modulated with a data train with an advantage of low cost, and a disadvantage of low reach and capacity performance. After binary modulation, a high bit may be transmitted as an optical signal level with more power than the optical signal level in a low bit. Often, the optical signal level in a low bit is engineered to be equal to, or approximately equal to zero. In addition to binary modulation, the data can be transmitted with multiple levels, although in current optical transport systems, a two level binary modulation scheme is predominantly employed.
Typical long haul optical transport dense wavelength division multiplexed (DWDM) systems transmit 40 to 80 10 Gbps (gigabit per second) channels across distances of 3000 to 6000 km in a single 30 nm spectral band. A duplex optical transport system is one in which traffic is both transmitted and received between parties at opposite ends of the link. In current DWDM long haul transport systems transmitters different channels operating at distinct carrier frequencies are multiplexed using a multiplexer. Such multiplexers may be implemented using array waveguide (AWG) technology or thin film technology, or a variety of other technologies. After multiplexing, the optical signals are coupled into the transport fiber for transmission to the receiving end of the link.
At the receiving end of the link, the optical channels are de-multiplexed using a de-multiplexer. Such de-multiplexers may be implemented using array waveguide (AWG) technology or thin film technology, or a variety of other technologies. Each channel is then optically coupled to separate optical receivers. The optical receiver is typically comprised of a semiconductor photodetector and accompanying electronics.
The total link distance may in today's optical transport systems be two different cities separated by continental distances, from 1000 km to 6000 km, for example. To successfully bridge these distances with sufficient optical signal power relative to noise, the total fiber distance is separated into fiber spans, and the optical signal is periodically amplified using an in-line optical amplifier after each fiber span. Typical fiber span distances between optical amplifiers are 50-100 km. Thus, for example, 30 100 km spans would be used to transmit optical signals between points 3000 km apart. Examples of in-line optical amplifiers include erbium doped fiber amplifiers (EDFAs) and semiconductor optical amplifiers (SOAs).
A duplex optical transport system is one in which voice and data traffic is both transmitted and received between parties at opposite end of the link. There are several architectures that support duplex operation in fiber optical transport systems. Each suffers from some limitation.
Long-distance transmission of data at high speed is increasingly dependent on optical fibers and efficient state-of-the-art light wave transmission systems. These transmission systems typically include, the optical fiber itself and among other things, end terminals, programmable optical amplifiers, optical spectrum analyzers, regenerators, and to be generically referred to as components in the transmission system. Besides transmitting customer data at high data rates, fiberoptic transmission systems that span large geographic distances must be able to send maintenance information or telemetry from one part of the system to another. In traditional regenerated systems, maintenance information is added as overhead to customer data which is transmitted on the fiber. Access to the overhead data is allowed at each regenerator site. The data can be provided to a local user or passed through the data collection station unchanged. In systems employing optical amplifiers, maintenance information is transmitted on a separate wavelength on the same optical fiber. Access to customer data is usually not possible because it is transmitted on a different wavelength.
Service providers who assemble and maintain light wave transmission systems, and network providers who manage networks of such optical transport systems are under increasing pressure to reduce costs. Equipment costs, maintenance costs and the cost of personnel to operate the transmission system all must be kept at a minimum in order for service providers to remain as competitive as possible. In current optical transport systems it is necessary to manually find and track the very large number of components, subsystems and modules that comprise the optical transport system. In order to manage, maintain and grow the optical transport system it is necessary to have an up-to-date, correct inventory of the various components, subsystems and modules that comprise the optical transport system. This inventory comprises a complete and correct listing of the type and quantity of the various components, subsystems and modules that comprise the optical transport systems. Human error combined with changing configurations currently lead to inefficient incorrect inventory process. Consequently there is a need for an optical transport system that automatically discovers the various components, subsystems and modules that comprise the optical transport systems. Such a capability is referred to as autodiscovery. Similarly, there is a need for an optical transport system that automatically inventories the various components, subsystems and modules that comprise the optical transport systems. Such a capability is referred to as autoinventory. In order to manage, maintain and grow the optical transport system it is additionally necessary to have an up-to-date, correct topology of the various components, subsystems and modules that comprise the optical transport system. This comprises a complete and correct arrangement of the connections and relative locations of the various components, subsystems and modules that comprise the optical transport systems. Human error combined with changing configurations currently lead to inefficient incorrect topology process. Consequently there is a need for an optical transport system that automatically determines the topology of the various components, subsystems and modules that comprise the optical transport systems. Such a capability is referred to as autotopology.
Provisioning a new channel to provide duplex operation between terminals that may be located in different cities is currently a tedious, labor intensive process. Consequently there is a need for an optical transport system that automatically provisions a new channel to provide duplex operation between distant terminals. Such a capability is referred to as auto-provisioning.
Certain physical characteristics of each optical fiber and the components used in the transportation system impose limitations on the data rate and transmission distance of any light wave transmission system. The performance of the components used, depends on, among other factors, the settings of the components, for example the gain setting on an in line amplifier. The performance of the fiber is based mainly on attenuation and chromatic dispersion properties of the fiber. The physical characteristics of each optical fiber cannot be readily adjusted for optimal performance. However, the components used in the transport system can be adjusted for optimal performance.
Attenuation is a primary limitation in the design of a light wave system. Attenuation is the loss of optical energy which is caused by impurities in the fiber, scattering and other phenomena. Loss of optical energy negatively affects information transmission by reducing the amplitude of the transmitted signal. Attenuation can be corrected, within limits, by regeneration or the addition of optical amplifiers along the path of the optical fiber to amplify the signal to replace lost optical energy. Further, the relative attenuation of a channel operating at wavelength as compared to a channel operating at a second wavelength is also important in an optical transport system. Different channels may see different relative attenuations because of the different spectral filtering of the different components, subsystems and modules that comprise the optical transport system.
Another important limitation is chromatic dispersion. Chromatic dispersion, or more precisely, group velocity dispersion, is caused by a variation in the group velocity in a fiber with changes in optical frequency. Chromatic dispersion causes the spreading of pulses in a light wave signal. Pulse spreading leads to timing problems and increased error rates.
In order to maximize the data transmission distance and speed, it is necessary to measure or characterize attenuation and chromatic dispersion for each fiber. After characterization, it is also necessary to correct for these limitations as far as is possible to maximize data transmission speed and distance. In the prior art, characterization takes place manually requiring a labor-intensive process of measuring each parameter at the time of installation. Consequently, there is a need for an optical transport system that automatically characterizes the physical properties of the fiber plant and other components, subsystems and modules that comprise the optical transport system. Such a capability is referred to as autocharacterization. After characterization, the system operator must currently manually correct for attenuation and chromatic dispersion by setting certain parameters on each amplifier specification. Consequently, there is a need for an optical transport system that automatically corrects the physical properties of the fiber plant and other components, subsystems and modules that comprise the optical transport system. Such a capability is referred to as autotuning. Autotuning will automatically adjust the control parameters of the optical transport system to account for changes in the number and character of the channels that are operational at a given time.
Attenuation is corrected for in the prior art by adjusting the gain and tilt of each amplifier in the system. The gain of each amplifier causes a linear difference in amplification over the range of wavelengths amplified. The linear difference is known as tilt. Additionally, stimulated Raman scatterings can cause significant power tilt in the fiber which is further amplified. Skill and experience of the operator are required to account for and correctly adjust each component of the transmission system for maximum efficiency at each wavelength and to compensate for tilt as much as possible. However, this prior art procedure is prone to human error and inconsistency.
Chromatic dispersion is corrected for in the prior art by manually measuring the dispersion which has taken place in the signal at any of the amplifier locations. “Compensators” are then fabricated which compensate for the dispersion measured by the operator and installed to correct the dispersion. Fabrication of compensators is time consuming and can take up to several weeks to complete. The process is also so specific that minor changes in fiber characteristics due to repairs and fiber aging can require a repeat of the process. Also, because each fiber is typically measured only once, changes in the system are largely unaccounted for during system operation, reducing the efficiency of the system and the maximum data rate over time.
Certain prior art systems have attempted to address these problems with varying success.
U.S. Pat. No. 5,914,794 to Fee, et al., entitled “Method of And Apparatus For Detecting and Reporting Faults In An All-Optical Communications System”, discloses a method and system for reporting and detecting faults in an optical communications systems. A system monitors each optical fiber and uses the optical supervisory channel to detect and report faults. However, Fee does not disclose or suggest a way to eliminate the time consuming necessity for manual adjustment of the transmission system.
U.S. Pat. No. 5,225,922 to Chraplyvy, et al., entitled “Optical Transmission System Equalizer”, discloses an invention which selectively equalizes the optical gain or optical signal noise ratios of channels of a wavelength multiplexed optical transmission system. The optical output powers and the signal to noise ratios are selectively equalized by adjusting the optical input powers through a controller connected to an end terminal of the transmission system. However, the Chraplyvy system has drawbacks. It is difficult to apply on long-haul systems and does not account for dispersion characterization or measurement. Additionally, the Chraplyvy system does not provide for adjustment of amplifier ripple as opposed to amplifier gain.
U.S. Pat. No. 5,940,209 to Nguyen, entitled “Interactive Optical Fiber Amplifier, System And Method”, discloses a system and method for selectively amplifying an optical signal, depending upon the amplification needs of the system. This invention adjusts amplifier gains controlled by remote systems so that an optical path in the system can be changed and automatic amplification provided. Nguyen has drawbacks, however, in that it requires a reflected power detector which adds to equipment costs and complexity. Additionally, the Nguyen invention does not address automatically adjusting amplifiers for ripple or remotely sensing chromatic dispersion.
U.S. Pat. No. 5,737,118 to Sugaya, et al., entitled “Optical Amplifying Apparatus”, discloses a computer controlled optical amplifying apparatus which includes an optical amplifying unit including an optical amplifying controller which controls the amplifier. The controller disclosed by Sugaya monitors the status of the optical amplifier to report abnormal occurrences in relation to the relaxation time of the amplifier. The Sugaya invention, as with the other prior art inventions, does not address the need for the automatic characterization and correction for chromatic dispersion or continued operation of the amplifiers during a fault sequence.
U.S. Pat. No. 6,317,231 to Al-Salameh, et al., entitled “Optical Monitoring Apparatus And Method For Network Provisioning And Maintenance”, provides for elimination of optical to electrical and electrical to optical signal conversion for monitoring certain maintenance functions of an optical network. The network controller provided analyzes the values of the optical signals to determine certain fault conditions and monitors certain channel power requirements. Additionally, the invention provides for monitoring signal noise ratio, channel continuity and network provisioning. Al-Salameh does not provide for automatic detection of chromatic dispersion or automatic adjustment of amplifier gain or tuning of components in the transmission system to eliminate the need for human intervention to correct system faults.
U.S. Pat. No. 6,163,392 to Condict, entitled “Distributed Intelligence Wavelength Division Multiplexed Network”, provides for a fiber-optic communication network in which processors associated with each network element periodically transmit identification and status information to other processors in the network. In addition a service channel carries diagnostic and span topology information that can be transmitted through each span. While Condict does provide for information to be transmitted on a reserved wavelength, the information is limited to identification and status information and routing data. Condict does not disclose or suggest a system that provides for automatic detection of chromatic dispersion, automatic adjustment of amplifier gain or tuning of components in the transmission system to eliminate the need for human intervention to correct system faults.
U.S. Pat. No. 6,359,729 to Amoruso, entitled “Optical Communication System And Component Control Architectures And Methods”, provides for control of optical components and network management. A component controller is configured to receive element instructions from an element manager and provides work function instructions to one or more work function controllers. The work function controller controls and monitors the work function pursuant to work function instructions provided by the component controller. Element instructions and other system information can be transmitted through the optical system using either a dedicated service channel or a mixed data channel carrying both communication traffic and system information. Amoruso does not disclose or suggest a system that provides for automatic detection of chromatic dispersion or automatic adjustment of amplifier gain or tuning of components in the transmission system to eliminate the need for human intervention to correct system faults.
Prior art systems suffer from the limitation that external measurement devices must be used by the operator to measure and correct for attenuation and dispersion. Also, the tuning of various components in the transmission system must be manually done by a system's operator. A further limitation of prior art systems is the length of time to restart the system after system failure. In some cases tuning and characterizing and correcting for attenuation and dispersion can take hours or days using the methods of the prior art.